Measuring spatial angles, trajectories, contours and movements as well as gravity anomalies signifies a difficult problem in the technical field, as can be seen from the following examples.
In structural engineering inclinations as a function of curved radii have to be measured for the construction of roads or railroads; similar problems exist in civil engineering in connection with the measurement and construction of tunnels, wells and pipes. For boring into the interior of the earth the course of the drill stems has to be defined and controlled. Elastic deformation characteristics, i.e. angular and positional changes as a function of exterior strain have to be considered in automotive engineering and aircraft construction. In wind tunnel techniques aerodynamic forces and moments have to be included as regards their dependence upon the angle of attack. In tests with aircraft and vehicles movements have to be traced with regard to position, velocity, attitude and heading as a function of time. In geodesy a tract of land is determined by measurements and plotted into maps starting from an astronomicly determined point of reference. Finally in connection with these measurements gravity anomalies are often of interest, as the direction of gravity and its anomalies affect astronomical measurements. Gravity anomalies can supply information on possible mineral resources.
Nowadays gyros and complete inertial systems (INS) are already being used for the resolution of the above mentioned problems. Because of the integration with time of the gyro and accelerometer signals (a*) for the computation of the angle or position, system deviations build up which increase with time and are due to the sensor's measurement deviations. In order to keep these deviations small, either high- capacity and therefore expensive systems are being used and/or calibration or aiding procedures.
Calibration procedures are implemented either before or after the measuring procedure itself, as known external references are then available. When using gyros or complete inertial systems, the calibration procedures are insofar disadvantageous, as they require a large amount of time. The computation of the parameters of an error model is implemented through integration with time, and the calibration accuracy increases with increasing time. Calibration procedures of that kind are, for example, known for the gyros of a three-axis stabilized satellite [German Specification DE 37 34941 C2] or for the initiation and/or recalibration of a subsidiary inertial navigation system [German Specification DE 3445463 A1]. A further disadvantage of the calibration procedure lies in the fact that the accuracy of the subsequent measurement series will be limited, if sensors are being used whose error parameters are a function of time. The calibration mentioned above procedures rely on a certain stability of those values in the course of the measurement procedure. This disadvantage can be seen in the case of the measurement procedures with gyros which are used for deep boring in the inner of the earth. In this case the temperature and thus drifting conditions change drastically and extremely for the gyros during the measuring process, and the calibration data obtained in the beginning or towards the end of the procedure are not representative for the whole measurement series.
At the present state of technical development the application of so-called aiding procedures comprises the use of known external reference data during the measuring process together with special mathematical algorithms (e.g. Kalman filter) [Gyroscopic Instruments and their Application to Flight Testing, AGARDograph No. 160, Vol. 15, September 1982, Chapter 8]. Compared with the calibration procedures mentioned above they can successfully be applied even if sensors of inferior quality, i.e. with instable error parameters are used. A disadvantage of this procedure is, of course, the provision of such external measurement data, which generally requires the availability of measuring instruments, or which is beyond any possibilities. While in the case of flight tests signals from radio navigation aids are available for positional aiding of inertial systems, those reference data are frequently not at hand for the positional measurement of drill stems. For geodetic measurements based on inertial systems (inertial geodesy) the aiding is based on accurately defined reference points and, in addition, on "zero velocity updating (ZUPT)", which means that the measurement vehicle is stopped and the velocity zero is fed into the computer as aiding information. It is true that the latter is an aiding method which can easily be implemented, but it requires an increased amount of time for the measurement process as a whole, and it is limited in its accuracy for the subsequent definition of the reference points, as the aiding does not comprise any external reference point measurements. Aiding information for the definition of gravity anomalies in a test area by means of an INS requires comprehensive and time consuming measurements at clearly defined reference points preliminary to the measurement series itself by means of gravimeters, and is therefore only rarely available. The invention constitutes however a valuable supplement to all the described aiding procedures.
In the U.S. Pat. No. 4,799,391 a procedure for the measurement of the curvature of pipelines is described using so-called pigs, which are taken along the pipeline by the medium transported as separating elements between the individual charges. Like an aircraft these pigs are equipped with an INS with three gyros, three accelerometers and a digital computer. For the definition of positions by means of an INS external measurements for the INS aiding are employed such as indicators along the pipeline, for instande magnetic anomalies in form of girth welds or similar markers. The measurments are taken at a high sampling frequency during the one time travel of the pig along the pipeline.
Further reference is made to commonly owned U.S. patent application Ser. No. 07/699,481.